During oil and gas well drilling, the drilling fluid density may be adjusted to balance pore pressure at all or most depths. While pumping fluids, the well bore pressures are typically higher than when the pumps are off. This pressure increase may be due to the friction of the drilling fluid as it flows up the well. The pressure fluctuations due to pumps-on versus pumps-off may cause over pressurization at certain zones in the well such that small fractures may be opened and fluid may be forced into these fractures at the higher pumps-on pressures. When the pumps are turned off, the pressure may drop and the formation at these high pressure zones can then potentially force fluids (or gas) back into the well. The result can be a cycle of transient loss of fluids while drilling followed by fluid (or gas) influx at pumps-off. Historically, this cyclic series of flows and losses is referred to as ballooning or breathing. The influx at pumps-off can be large and is often misinterpreted as a “kick” which is a result of natural pore pressure being higher than the surrounding fluid pressure. The driller's actions for a “kick” (e.g. shut in the well and increase drilling fluid density) can sometimes exacerbate ballooning. It is therefore often important to quickly diagnose an initial influx as either the result of a ballooning cycle or as a “kick”.
Traditionally, drillers have relied on human observations of prior fluid loss and generally adopted procedures that may require well shut in and pressure measurements. Inaccurate assessment of prior fluid losses can lead to errors and misdiagnosis of influx as kicks. Drillers sometimes react to ballooning with kick control procedures and thus exacerbate ballooning. This can ultimately lead to an underground blow-out (influx at one depth and fluid losses as a separate depth), with possible environmental damage and loss of the well. What is needed is a way to more accurately determine if well influx is the result of formation ballooning or a kick. It may also be desirable to automate the diagnosis of ballooning by processing real time data, so that drillers may take the correct actions as quickly as is desirable.
Careful analysis of fluid flows and volumes, throughout the time interval from several minutes prior to pumps-off until several minutes after pumps-on, may allow for an automatic assessment of the confidence that fluid losses have initiated and/or begun to increase at pumps-on. This trend in fluid loss is then to be carefully monitored and may be combined with one of many potential influx detection algorithms. After pumps-off, the fluid flow-out patterns may also be processed to determine if flow-out is gradually decreasing (i.e. consistent with ballooning), or is steady, or increasing (i.e. consistent with a “kick”). When influx is first detected, that event may be combined with prior fluid loss information and/or previous flow-out patterns to provide a more accurate assessment of whether the initial influx is due to well ballooning or a kick.
Advanced processing may be applied to flow and volume measurements to allow accurate trend and/or jump detections of changes in well fluid flow (e.g. differences in flow-out and flow-in) at pumps-off and/or pumps-on. Comparison of the differences at these two ends of the pumps-off and pumps-on on cycle may yield new information not previously available.